Quo Vadis Nex?

Started May 16, 2011 | Discussions thread
seachicken2000
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Re: Quo Vadis Nex?
In reply to blue_skies, May 17, 2011

To the best of my knowledge, you can make a lens longer than its focal length with special optics, but you cannot make it shorter than its focal length, unless you resort to mirror (tele) lenses, 'collapsible lenses' and sorts.

The good news is that I think this not only possible, but necessary in telephoto lens design. I vaguely remember studying this a long time ago.

From: http://en.wikipedia.org/wiki/Telephoto_lens

If a camera lens were to be constructed from a single lens of 500 mm focal length, then when the lens is focused on an object at infinity, the lens will be 500 mm away from the focal plane where the film or sensor is....

As the focal length of such lenses increases, the physical length of lens becomes inconveniently long. But such lenses are not telephoto lenses, no matter how extreme the focal length. They are simply known as long-focus lenses.[1] A telephoto lens works by having the outermost (i.e. light gathering) element of a much shorter focal length than the equivalent long-focus lens and then incorporating a second set of elements close to the film or sensor plane that extend the cone of light so that it appears to have come from a lens of much greater focal length.

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